Method of inspecting an operation of sealed closure by welding the end of a filling channel traversing the upper plug of a nuclear fuel rod

ABSTRACT

A method of inspecting an operation of sealed closure by welding an end opening of a filling channel axially traversing an upper plug for closing the cladding of a fuel rod for a nuclear reactor, the cladding of the rod containing a plurality of pellets of nuclear fuel stacked in the axial direction of the cladding and two closure plugs, one of the plugs or the upper plug being traversed axially by the channel for filling the cladding of the rod with an inert gas and the sealed closure by welding of the filling channel of the upper plug being carried out after filling the cladding with inert gas, in a filling apparatus, by melting central part of the end of the upper plug adjacent to the opening of the filling channel, this method allowing for inspection of the conditions for implementing and carrying out the sealed closure of the upper plug by welding, efficiently and without extending the time needed for the manufacture of the fuel rod.

FIELD OF THE INVENTION

The present invention relates to a method of inspecting an operation ofsealed closure by welding the end of a filling channel traversing theupper plug of a nuclear fuel rod.

BACKGROUND INFORMATION

Nuclear reactors, such as nuclear reactors cooled by pressurized water,comprise a core consisting of fuel assemblies in which energy in theform of heat is produced during operation of the reactor.

Each of the fuel assemblies generally consists of a bundle of mutuallyparallel fuel rods held in a framework of the fuel assembly. Each of thefuel rods comprises a tubular padding made of a material which weaklyabsorbs neutrons such as a zirconium alloy, in which nuclear fuelpellets are stacked, for example sintered pellets of UO₂ uranium. Thetubular cladding is closed at its ends by plugs, each of which comprisesa cylindrical part which is engaged coaxially in an end part of thecladding. The plug and the cladding are then fastened one to the otherby welding around a circular line located in a plane which issubstantially perpendicular to the axis of the cladding and of the plug.

One of the two plugs for closing the cladding of a fuel rod, which iscalled the upper plug, because it closes the upper end of the rod insidethe fuel assembly in the service position of the fuel assembly in thecore of the nuclear reactor, is traversed axially by a channel, whereinthe cladding of the rod is filled, around the nuclear fuel pellets, byan inert pressurized gas such as helium which protects the fuel pelletsagainst oxidation and promotes heat exchange between the pellets and thecladding of the rod when the rod is in service in the core of thenuclear reactor.

The manufacture of the fuel rods requires numerous successive operationsin order to fill the cladding with the fuel pellets, to place and weldthe plugs and to introduce an inert pressurized gas such as helium intothe cladding sealed shut by the plugs, and to close the filling channelof the plug, after filling. Numerous inspections must be carried out atall steps of the fuel rod manufacture, so as to attain fuel rods whichare completely free of defects.

In particular, sealing the closure by welding the filling channel of theupper plugs must be subject to rigorous inspection.

The filling with pressurized helium of the cladding of the rodscontaining the fuel pellets and sealed shut by the plugs is carried outin a filling apparatus in which the upper end part of the fuel rod,comprising the upper plug traversed axially by the filling channel, isinserted. The end of the upper plug, on which the filling channelemerges in the form a circular inlet opening extended axially by achamfered part of the filling channel, is placed so as to be able toengage with a valve for closing and opening the chamfered end part ofthe filling channel. In the open position of the valve, the aircontained inside the fuel cladding is evacuated, then the rod is filledby pressurized helium entering inside the cladding via the fillingchannel of the upper plug. Finally, in the filling apparatus, the end ofthe filling channel is sealed shut by a weld obtained by melting acentral part of the plug adjacent to the end part of the fillingchannel. Generally, the material of the plug is melted to carry out theweld under the effect of a laser beam directed axially on the chamferedinlet part of the filling channel. The weld obtained by a pulsed laseris generally formed from three successive spot welds, in order toincrease the safety and the production quality of the closure. Thechamfered inlet part of the filling channel is generally known by theterm “seal weld”.

The weld is carried out under satisfactory condition only if the axis ofthe laser beam for melting the material of the plug, along the peripheryof the chamfered inlet part of the filling channel, is centeredaccurately with respect to the circular inlet opening of the seal weldand if this circular opening has a diameter the size of which ispredetermined and defined very accurately.

Furthermore, after welding, the quality of the spot welds can beinspected, in order to determine whether the fuel rod compliesperfectly.

The inlet opening of the filling channel or pressurization hole ispositioned manually and with simple visual monitoring, before thewelding operation. The mechanical adjustment for manually controllingthe positioning of the plug and of the pressurization hole cannot enablethe perfect positioning of the pressurization hole to be guaranteed inall cases during welding. There are possibilities of maladjustment orblockage of the rod, which result in poor positioning. In addition, thevisual monitoring of the positioning of the filling channel does notmake it possible to obtain a highly efficient adjustment either.

The quality of the spot weld is inspected by X-ray scanning. Thisinspection makes it possible to detect porosities in the weld but doesnot allow the position and size of the weld to be verifiedsatisfactorily.

Furthermore, the inspection must be carried out on a separate station ofthe filling apparatus, which makes the rod manufacture operations morecomplex and longer.

Where the sealing weld is carried out by laser welding, the quality ofthe laser weld depends on the geometry of the chamfer of the inlet partof the filling channel; in particular, it is necessary to verify thatthe chamfer has an inlet opening whose diameter, which constitutes themaximum diameter of the chamfer, complies with extremely strict sizerequirements.

SUMMARY

The aim of the invention is therefore to propose a method of inspectingan operation of sealed closure by welding an end opening of a fillingchannel axially traversing an upper plug for closing the cladding of afuel rod for a nuclear reactor, the cladding of the rod containing aplurality of pellets of nuclear fuel stacked in the axial direction ofthe cladding and two closure plugs, one of the plugs or the upper plugbeing traversed axially by the channel for filling the cladding of therod with an inert gas and the sealed closure by welding of the fillingchannel of the upper plug being carried out after filling the claddingwith inert gas, in a filling apparatus, by melting central part of theend of the upper plug adjacent to the opening of the filling channel,this method allowing for inspection of the conditions for implementingand carrying out the sealed closure of the upper plug by welding,efficiently and without extending the time needed for the manufacture ofthe fuel rod.

With this aim, prior to the sealed closure of the filling channel, thefuel rod being in the position for filling and for sealed welding of theupper plug in the filling apparatus, images are acquired of the end ofthe plug on which the substantially circular inlet opening of thefilling channel emerges, and in order to obtain a digitized image, theposition of the center of the circular inlet opening of the fillingchannel is determined with respect to a reference position and thediameter of the inlet opening of the filling channel by analyzing thedigitized image. It is thereby deduced whether it is possible to weldthe filling channel. Where the sealed closure of the filling channel iscarried out by welding, images are acquired of the end of the upper plugafter welding and the presence and the position of a weld for sealedclosure of the filling channel are determined.

BRIEF DESCRIPTION OF THE DRAWINGS

In order for the invention to be properly understood, the implementationof the inspection method according to the invention will now bedescribed, by way of example, with reference to the appended figures, inthe case where the filling channel of an upper plug of a fuel rod issealed shut by laser welding.

FIG. 1 is a view in partial axial section of the upper end of a fuelrod, in a manufacturing phase before it is filled with inert gas and theupper plug is sealed shut.

FIG. 2 is a view in partial axial section similar to the view of FIG. 1illustrating the upper end part of the rod after the upper plug issealed shut by welding.

FIG. 3 is a schematic view of a station for filling with inert gas andfor the sealed closure by laser welding of the upper plug of fuel rodsand of a device for inspecting the weld of the upper plugs of the fuelrods.

FIG. 4 is a explanatory image illustrating the successive searchoperations performed before welding in order to implement the methodaccording to the invention.

FIG. 5 is a graph illustrating the gray level of points of a column orrow of the image which is obtained during the inspection, before weldingthe filling channel of the upper plug of a fuel rod.

FIG. 6 is an example of an image appearing on the screen of theinspection device on implementing the method according to the inventionbefore welding the filling channel of the upper plug of a rod.

FIG. 7 is an example of an image appearing on the screen of theinspection device on implementing the method according to the inventionbefore welding the filling channel of the upper plug of a rod.

FIG. 8 is an example of an image appearing on the screen of theinspection device on implementing the method according to the inventionbefore welding the filling channel of the upper plug of a rod.

FIG. 9 is an explanatory image illustrating the successive searchoperations carried out on implementing the method according to theinvention, after welding the filling channel of the upper plug of a fuelrod.

FIG. 10 is a graph illustrating the gray level along a row or a columnof the image supplied by the inspection device, after welding thefilling channel of the upper plug of a rod.

FIG. 11 is an example of an image appearing on the screen of theinspection device, as a result of inspecting the weld of the fillingchannel of an upper plug of a fuel rod.

FIG. 12 is an example of an image appearing on the screen of theinspection device, as a result of inspecting the weld of the fillingchannel of an upper plug of a fuel rod.

FIG. 13 is an example of an image appearing on the screen of theinspection device, as a result of inspecting the weld of the fillingchannel of an upper plug of a fuel rod.

DETAILED DESCRIPTION

The upper end part of a fuel rod for a pressurized-water nuclearreactor, denoted generally by the reference 1, is illustrated in FIG. 1and in FIG. 2.

The rod 1 comprises, in particular, tubular cladding 2 made of zirconiumalloy which contain fuel pellets 3 and which is closed at its upper end,represented in FIG. 1 by a plug 4 constituting the upper plug of thefuel rod.

The plug 4 is generally made up of zirconium alloy and comprises a part4 a which is engaged, virtually without clearance, in the end part ofthe bore 2. After filling the cladding 2 with fuel pellets 3, theclosure plugs of the cladding, such as the upper plug 4, are engaged inthe end parts of the cladding, the sealed attachment of the plug beingprovided by a weld line such as 5 made by melting the material of theplug and of the cladding, along a circular line.

A spring for holding the fuel pellets is inserted between the upper plug4 and the end of the column of fuel pellets 3.

The upper plug 4, which is axisymmetric about an axis 6, comprises anend part 4 b opposite, in the direction of the axis 6, to an end 4 aengaged in a coaxial arrangement inside the cladding 2. The outside ofthe end part 4 b of the plug 4 has, successively in the axial direction5, from the upper end of the plug, a frustoconical plug, a shoulder anda cylindrical part along which the diameter of the plug is at a minimum.The minimum-diameter part of the plug and the shoulder make it possibleto engage and to fasten the jaws of a gripper tool, during fuel rodmaintenance operations.

The plug 4 is traversed, in the direction of the axis 6, along a wholelength, by a channel 7 which will be denoted subsequently as a fillingchannel, this channel 7 making it possible to fill the cladding of therod with an inert pressurized gas, as will be explained below. Thefilling channel 7 of the plug 4 comprises several successive parts, thediameters of which generally decrease from the end 4 a of the plugengaged in the cladding 2 to the outer end part 4 b of the plug.

At an upper end or outer end, the channel 7 comprises a terminal part 7a of frustoconical shape or a seal weld flared from the bottom upwards,i.e. from the inside to the outside of the plug, which emerges on theupper end surface 8 of the plug along a circular inlet opening, thecenter of which is on the axis 6 common to the plug and the cladding 2.

The seal weld 7 a of the channel 7, along which the channel 7 is closedby a weld, provides the junction between a part 7 b of the channel 7 ofminimum cross section and the circular inlet opening of the channel 7 onthe upper end surface 8 of the plug 4.

The small-diameter part 7 b may have a diameter of about 0.6 mm and thecircular inlet opening of the terminal frustoconical part 7 a a diameterof about 1.3 mm.

As illustrated in FIG. 2, after having filled the cladding of the rodwith an inert pressurized gas, such as helium, the channel 7 is sealedshut in its upper terminal part, by a weld 10 obtained by melting thematerial of the plug in the central region of its upper end part, at theperiphery of the seal weld 7 a and consisting, for example, of threelaser spot welds.

In FIG. 3, the upper plug 4 of the fuel rod 2 is filled and sealed in afilling and welding apparatus 9.

The filling and welding apparatus mainly comprises a enclosure, one wallof which comprises an opening for the passage of a fuel rod 2 in ahorizontal arrangement, the upper end of the rod being inserted into theenclosure, so that the upper plug 4 of the fuel rod 2 engages with avalve 11, the closure member of which makes it possible to close or toopen the terminal end of the filling channel of the plug.

The enclosure of the filling and welding apparatus 9 comprises a partlocated opposite the part for introducing the rod 2, on the walls ofwhich are mounted a laser beam welding device 12 and an optical assembly13 making it possible to position the laser beam in order to weld thesealed closure of the filling channel of the upper plug 4 of the fuelrod 2 and which is also used to implement the inspection method byanalyzing the optical image according to the invention.

A mirror 14 for reflecting and focusing the laser welding beam emittedby the laser welding device 12 is placed inside the second part of theenclosure 9. The device 12 comprises an optical fiber 15 connected to alaser source and a collimator 16, so as to send, by the reflection andfocusing mirror 14, a laser beam of axial direction, on the end part ofthe filling channel 7 constituting the seal weld.

To allow a sealed closure of the filling channel by a weld 10 undersatisfactory conditions, the laser beam may be directed accurately ontothe center of the circular inlet opening of the filling channel 7 of theupper plug 4. In order to adjust the position of the laser welding beam,the optical illumination and adjustment assembly 13, which comprises anillumination device 17 and an optical sighting assembly comprising areticule, is used so as to identify the direction along which the lasershot is sent.

In the case of methods of adjusting the welding according to the priorart, adjusting the position of the upper plug of the rod is carried outmanually and verified visually.

In the case of the method according to the invention, the illuminationdevice 17 for sighting and adjusting the laser shot is used in order toinspect, on digitized images, the position of the center of the inletopening of the filling channel with respect to the laser beam and thecompliance of the inlet opening, in order to determine whether thewelding of the filling channel, after the rod is filled with pressurizedhelium, can be carried out satisfactorily.

The weld, consisting of spot welds, is also inspected by analysis ofdigital images.

For this purpose, a digital camera 18 a connected to a microcomputer 20comprising a screen for displaying images 19 is combined with an optic18 directed in the axis of the enclosure 9 along which the fuel rod 2 isengaged. The microcomputer 20 comprises a video acquisition card and adigital input/output card enabling the microcomputer 20 to communicatewith a controller managing the laser welding device 12.

The microcomputer 20 receives inspection orders from the controller, viathe digital input/output card, and verdicts established from the resultsof the inspection are sent by the microcomputer 20 to the controller forcontrolling the laser welding device 12. Information relating to the rodand the welding conditions is transmitted to the microcomputer 20 by thecontroller of the laser welding device 12.

A first step of the method for inspecting the sealed closure by weldingthe filling channel of the upper plug of a rod is in determining theposition and the size, i.e. the diameter of the inlet opening 8 a of thefilling channel 7, consisting of the circular outline along which thefilling channel 7 opens on the end surface 8 of the upper plug 4 of therod. This inlet opening constitutes the large diameter end of thechamfer of the terminal part 7 a of the filling channel.

The inspection is performed on the filling and welding apparatus asdescribed, during a phase in the course of which the fuel rod is filledwith pressurized helium, the valve 11 being open.

An image is acquired of the end surface 8 of the plug and of the inletopening 8 a of the terminal part 7 a of the filling channel using theoptic 18 and the digital camera 18 a, the optical image obtained beingdigitized by the digital camera 18 a and transmitted in digital form tothe inspection microcomputer 20.

The image obtained may be made visible on the screen 19 of themicrocomputer 20, as shown, for example, in FIGS. 6, 7 and 8.

The search operations performed on the circular opening of the seal weldwill be explained with respect to FIG. 4.

As a result of illuminating the end of the upper plug of the rod in anaxial direction, the end surface 8 of the rod appears as a light regionand the terminal part 7 a of the filling channel 7 as a dark region, thelight region and the dark region being separated one from the other by asubstantially circular line 8 a constituting the inlet end of the sealweld.

Analysis of a digital image of the end part of the upper plug allowsdetermination of the position of the center of the circular opening 8 awith respect to the center of a parameterizable sighting marker, markingthe position of the laser beam with which the closure is sealed bywelding the filling channel 7. Ideally, the position of the sightingmarker and the position of the center of the inlet opening of the plugshould be coincident.

The sighting marker 21 comprises a vertical axis and a horizontal axiswhich define the center of the sighting marker. It is positionedvisually when the laser welding station is correctly adjusted.

The edges of the opening 8 a are sought on the image along thehorizontal and vertical axes of the reference sighting marker 21.

The edges of the hole 8 a are sought along the horizontal axis of thesighting marker 21 or first axis. A number N is chosen, whichcorresponds to the number of rows of the image which will be used oneach side of the reference 21 in order to define a mean row along whichthe edges of the opening 8 a will be sought.

FIG. 5 illustrates the variations of the gray levels between the blackand the white, along the mean row determined above. The distances on theX-axis, i.e. along the row of the image, are expressed in image pointsor pixels.

A parameterized threshold value is chosen.

The threshold value is illustrated by the straight line 22 of FIG. 5.

The right edge of the image then the left edge are determined along themean row and by comparing the pixels to the threshold value.

From the right edge and from the left edge defined on the graph, theposition of the center of the hole of the seal weld is determined, forexample with respect to an edge of the image as illustrated by thesegment 23 and the diameter of the seal weld as shown by the segment 24.The edges of the hole of the seal weld are then sought along a secondaxis consisting of the vertical axis 25 (see FIG. 4) passing through thecenter previously defined or the first center.

The edges of the inlet opening of the seal weld are determined in thesame way as above, but using the columns of the image instead of therows. In this way, a second center of the circular inlet opening and thediameter along the vertical axis are defined.

The edges of the circular inlet opening 8 a of the seal weld are thendetermined, along the horizontal axis 26 or third axis passing throughthe second center defined by the search on the vertical axis. Theposition of the center of the circular opening 8 a and the diameter ofthe circular opening along the horizontal axis are deduced therefrom,this third determination of the center of the opening supplying theactual center taken into account.

The values obtained for the diameter along the vertical axis 25 andalong the horizontal axis 26 are checked for consistency, that is to saythat they do not differ by a value greater than a chosen thresholdvalue.

The pixels are then converted into millimeters and the diameter of thecircular opening 8 a is compared with threshold values defined by the“minimum diameter and maximum diameter” parameters of the opening 8 a.

The position of the center of the circular opening 8 a defined by itsdistance to the center of the sighting marker 21 is compared with athreshold value corresponding to a “tolerance of center position”parameter. The results are displayed on the screen of the microcomputer20, as shown in FIGS. 6, 7 and 8. In each case, a first value isdisplayed in millimeters corresponding to the position of the center ofthe circular opening 8 a, a second value is displayed in millimeterscorresponding to the diameter of the circular opening 8 a and a verdictindicating the compliance or the noncompliance of the measured values isdisplayed. Where the measured values comply, an execution command istransmitted to the controller of the laser beam welding device 12.Otherwise, a blocking command is transmitted.

In the case of FIG. 6, the values identified comply, the distancebetween the center of the opening 8 a and the center of the sightingmarker is less than a chosen threshold value and the diameter measuredbeing between the acceptable minimum diameter and the acceptable maximumdiameter.

In FIG. 7, the opening 8 a is offset with respect to the center of thesighting marker 21, such that the distance between the center of theopening 8 a and the center of the sighting marker is greater than thechosen threshold value. A verdict of noncompliance is thereforetransmitted. On the other hand, the diameter in this case is between themaximum and minimum threshold values.

In FIG. 8, the diameter measured on the image of the opening 8 a is lessthan the minimum threshold value. A verdict of noncompliance istransmitted. In addition, the position of the center of the opening 8 ais slightly offset with respect to the position of the sighting marker,the distance between the center of the opening 8 a and the center of thesighting marker is however less than the threshold value.

A fault verdict is also transmitted when it has not been possible tomark the edges of the opening 8 a in the course of the searches asdescribed above.

In the case of a verdict of compliance, a welding execution command issent to the controller of the laser welding device 12. The welding isperformed by the pulsed laser which melts the metal of the upper plug ina peripheral region of the terminal part 7 a or seal weld of the fillingchannel 7. A spot weld is produced closing off the inlet of thesmall-diameter part 7 b of the filling channel 7, then two successivepulses are produced to form the weld 10.

On its upper surface, the weld 10 is in the shape of a dish or craterdue to the flow and to the deposition of the metal in the hollow centralpart of the plug.

After welding, since the fuel weld is in position in the filling andwelding apparatus 19, it is possible to inspect for the present andcharacteristics of the spot welds.

For this purpose, the end surface of the rod 8 is illuminated, using theillumination apparatus 17, thereby sending light in an axial directiononto the surface of the rod, and images are acquired of the end surface8 of the rod using the optic 8 and the digital camera 8 a.

The digitized image is transmitted to the microcomputer 20.

The image of the end part of the plug can be displayed on the screen, asillustrated, for example, in FIGS. 11, 12 and 13.

The search operations performed on the spot weld will be explained withrespect to FIG. 9.

On the image, the end surface 8 of the plug and the reflection 27produced by the light reflected by the crater of the central part of thespot weld 10 appear as the light part.

By analyzing the digitized image, the distance from the center of thereflection 27 to the center of the parameterizable sighting marker 21,which is shown on the screen, is determined.

The position of the center of the reflection 27, that is to say thedistance from this center to the center of the sighting marker, iscompared with a “tolerance of center position” parameter and thediameter of the reflection is compared with threshold values defined by“minimum and maximum diameter” parameters.

The horizontal mean rows are constructed around the reference, then amaximum number of pixels greater than the threshold, and which bepositioned in the reflection, are determined.

The edges of the reflection 27 are then determined along a first axisconsisting of the horizontal axis passing through the position foundabove. For this purpose, a mean row is defined, in the way indicatedabove and a graph is drawn representing the variation of the gray levelsalong the mean row, as shown in FIG. 10.

A threshold value, represented by the horizontal straight line 28 inFIG. 10, is defined.

The right edge and the left edge of the reflection are determined bycomparing the successive pixels along the mean row with the thresholdvalue. A first position of the center of the reflection is calculatedwith respect to an edge of the image, this position being represented bythe segment 29 in FIG. 10.

A first diameter of the reflection between the right and left edges isalso calculated, this diameter being represented by the segment 30 inFIG. 10.

The edges of the reflection along a second axis consisting of a verticalline passing through the first center defined above are determined bycarrying out a search identical to the previous one but on the columnsof the image.

The consistency of the values obtained are verified for the diameter ofthe reflection 27 along the first and second axes.

The pixels are converted into millimeters and the diameter is comparedwith the “minimum and maximum diameter” threshold values and theposition, with the “tolerance of center position” parameter.

The results are displayed on the screen of the microcomputer, as shownin FIGS. 11, 12 and 13.

In each case, one of the items of information displayed in digital formrelates to the position and the other to the diameter of the reflection.The position of the reflection corresponds to the distance inmillimeters between the center of the reflection and the center of theparameterizable sighting marker forming the reference. The position ofthe center of the reflection corresponds to the position of the centerof the spot weld.

In the case of FIG. 11, both the position and the diameter of thereflection are satisfactory. A verdict of compliance is then displayed.

In the case of FIG. 12, the center of the reflection 27 is offset withrespect to the center of the reference 21 by a value greater than thepredetermined threshold value.

A verdict of noncompliance is therefore transmitted. However, thediameter of the reflection is satisfactory, that is to say between theminimum admissible value and the maximum admissible value.

In the case of FIG. 13 there is no reflection 27, which results in theabsence of the spot weld in the seal weld of the filling channel 7 ofthe upper plug of the rod. A verdict of noncompliance is transmitted.

In the case where a verdict of compliance is transmitted, the fuel rodmay be considered as correctly welded and may be accepted at the end ofmanufacturing.

The method according to the invention also makes it possible todetermine the depth over which the weld has been carried out and, inparticular, when only the first spot weld has been carried out, a verysmall reflection coming from the bottom of the seal weld is observed. Inthis case, the absence of the second and third spot welds is easilydetected from the measurement of the diameter of the reflection.

Examination of the graph giving the gray levels along a reference linepassing through the reflection of the spot weld also makes it possibleto determine the position of the center of the dish of the spot weldwhich is denoted by the reference 31 in FIG. 10.

Analysis of the optical images produced by the microcomputer 20 iscarried out using software.

The invention makes it possible, in the apparatus for filling andsealing the rod, to inspect the diameter of the opening of the seal weldof the upper plug of the rod and its positioning with respect to thelaser welding beam, so as to determine whether or not it is possible tocarry out the sealed closure by welding the rod filled with pressurizedhelium.

Secondly, the invention also makes it possible to verify that the spotweld has been carried out in a compliant manner. All the operations arecarried out in parallel on the filling and welding apparatus.

The method according to the invention makes it possible to avoid anyoperation of maintaining the rods between the filling and sealed closureapparatus and an inspection station and the verdict relating to thecompliance of the spot weld is available from the end of the weldingoperation.

Information relating to the operation as a whole (positioning, weldingand inspection) may be saved on a hard disk and can be exploitedsubsequently in the form of databases.

Finally, the illumination used to acquire images is a standardillumination which can be obtained by market available items.

The invention is not limited strictly to the embodiments which have beendescribed.

It is possible to process the digitized images of the end surface of theupper plug before and after welding, by methods other than those whichhave been described, for example for determining the edges of the inletopening of the filling channel and of the reflection of the spot weld.

Finally, the method according to the invention is applicable to anynuclear fuel rod comprising an upper plug traversed by a channel forfilling the rod with an inert pressurized gas.

What is claimed is:
 1. A method of inspecting an operation of sealedclosure by welding an end opening of a filling channel axiallytraversing an upper plug for closing a cladding of a fuel rod for anuclear reactor, the cladding of the rod configured to contain aplurality of pellets of nuclear fuel stacked in an axial direction ofthe cladding and two closure plugs, at least one of the plugs and anupper plug traversed by the channel for filling the cladding of the rodwith an inert pressurized gas and the sealed closure by welding of thefilling channel of the upper plug after filling the cladding with theinert pressurized gas, in a filling apparatus, by melting an end centralpart of the plug adjacent to the opening of the filling channelcomprising: prior to the sealed closure of the filling channel,acquiring images of an end surface of the plug on which the fillingchannel emerges by the substantially circular inlet opening to obtain adigitized image, wherein the rod is in a position for filling and forsealed welding of the upper plug in the filling apparatus and whereinthe inlet opening is configured to be substantially circular;determining a position of a center of the circular inlet opening of thefilling channel with respect to a reference position and a diameter ofthe inlet opening of the filling channel through analyzing the digitizedimage; deducing whether it is possible to weld the filling channel; andacquiring images of the end of the plug after welding and determining apresence and position of a spot weld for sealed closure of the fillingchannel, where the sealed closure of the filling channel is carried outby welding.
 2. The method according to claim 1, wherein the referenceposition consists of a center of a reticule comprising a horizontal axisand a vertical axis, the position of the center of the reticulecorresponding to a position for adjusting a welding arrangement forcarrying out the sealed closure by welding of the filling channel of theplug, the method further comprising: seeking the edges of the inletopening of the filling channel on the end surface of the plug along afirst axis of the reticule; deducing a first position of the center ofthe circular inlet opening of the filling channel and a first value ofthe diameter of the inlet opening; seeking the edges of the openingalong a second axis perpendicular to the first axis of the reticulepassing through the center deduced position; deducing a second positionof the center of the circular opening of the filling channel and asecond value of the diameter of the circular inlet opening of thefilling channel; seeking the edges of the circular opening along a thirdaxis perpendicular to the second axis passing through the secondposition of the center; deducing a third position of the center of thecircular opening and a third value of the diameter of the circularopening, the third position of the circular opening considered as anactual center of the opening; comparing the second value of the diameterand the third value of the diameter to deduce a consistency of thesecond and the third parameter values considered as parameters of thecircular opening; determining a distance between the third position ofthe center of the opening and the center of the reticule; and comparingthe distance calculated between the centers of the circular opening andof the reticule together with the calculated value of the diameter ofthe circular inlet opening of the filling channel to threshold values todetermine a compliance of the inlet opening of the filling channel andof a filling channel position and a possibility of carrying out sealedclosure by welding.
 3. The method according to claim 2, wherein theedges of the inlet opening of the filling channel are sought along eachof the axes from a graph providing gray levels along at least one of themean gray levels along N rows and N columns of the digitized imageparallel to the axis along which a search is carried out and which arelocated on each side of the axis.
 4. The method according to claim 3,wherein the edges of the circular inlet opening of the filling channelare determined by using a threshold value of the gray levelsconstituting a mean between the gray levels of the image of the fillingchannel and the gray levels of the surface of the upper plug around thecircular inlet opening of the filling channel.
 5. The method accordingto claim 1, wherein a reflection of the spot weld, having a central partin the shape of a crater reflecting light directed axially is sought onthe digitized image of the end surface of the plug after carrying outthe sealed closure of the filling channel of the upper plug, anddetermining a position of the center and the size of the reflection. 6.The method according to claim 5, wherein the position of the center ofthe reflection is determined with respect to a center of a reticuledefined by two horizontal and vertical axes, respectively, on thedigitized image, corresponding to a welding position and a diameter ofthe reflection, and in that the distance from the center of thereflection to the center of the reticule and a calculated diameter ofthe reflection are compared to threshold values to define whether theupper plug of the fuel rod is compliant after welding.
 7. The methodaccording to claim 6, further comprising: seeking the edges of thereflection along a first axis of the reticule; deducing a first positionof the center and a first value of the diameter of the reflection;seeking the edges of the reflection along a second axis perpendicular tothe first axis passing through the first center of the reflection;deducing a second position of the center and a second value of thediameter of the reflection; comparing the first and second diameters inorder to verify a consistency of the values obtained; determining adistance between the second position of the center of the reflection andthe center of the reticule; and comparing a distance between the centerof the reflection and the center of the reticule and the calculateddiameter of the reflection to threshold values to determine a complianceof carrying out the sealed closure by welding the filling channel of theupper plug of the rod.
 8. The method according to claim 7, wherein theedges of the reflection are sought along each of the first and secondaxes by determining a graph providing grey levels along a mean rowparallel to the first and second axes respectively, corresponding to amean of the gray levels along at least one of several rows and columnsof the digitized image which are parallel to one of the first and secondaxes respectively and placed on each side of one of the first and secondaxes.